1. Field of the Invention
This invention is concerned with a method of growing macrocrystal ingots of inorganic salts, metals and solid solutions from a stratified melt. More particularly, the invention is directed to the growing of alkali metal or alkaline earth metal halides for use in optical and scintillation applications.
2. Description of the Prior Art
A well-known method of growing crystals involves establishing and maintaining a quiet melt of a feed crystal in an enclosed container, creating a thermal gradient surrounding the container with the heat passing vertically downward through the melt, and thereafter instituting the growth of the crystal from the bottom of the melt upward toward the top surface. This method is referred to as the Bridgmann procedure for growing metals, and the Stockbarger and Stober method for growing crystals from salts and solid solutions. In Stockbarger growth, fresh crystalline feed stock often with some recycled stock, contained in a non-reactive crucible is placed in a furnace on a vertically movable elevator. The furnace has two heating sections separated by a baffle. During growth, the upper section is heated to a temperature of about 50.degree. to 150.degree. C. above the melting point of the crystal while the lower section is maintained at a temperature below the melting point. Thus, the flow of heat is from the upper section downwards through the melt, and the inferface into the grown crystal and then into the cooler lower section. The temperature of the melt during this period of growth is higher at its upper surface than at the interface.
During meltdown of the charge, the crucible is positioned in the upper section of the furnace after which the air is evacuated from the furnace and is replaced with a gas such as nitrogen which will not react with the salt and which will serve to suppress evaporation of the melt. To this gas is often added a small amount, less than 10%, of a specific gas such as CCl.sub.4 which reacts with an impurity known to exist within the melt.
After the salt is completely melted and a desirable temperature is established in the melt, the crucible is slowly lowered in the furnace past the baffle into the cooler lower section. The salt slowly crystallizes from the bottom of the melt on up as the temperature in the charge drops below the melting point in the vicinity of the baffle. The melt to crystal interface remains relatively stationary in horizontal proximity to the baffle with the slow downward movement of the crucible.
The temperature gradient in the melt along with interface position is maintained in a steady state condition by close control of power to the heaters. This also means a quiet melt, devoid of large convection currents, stratified as to temperature, density, dissolved gas concentration and in some instances, dopant concentrations during the growth process. Sensitivity and quality of the power control instrumentation is important for holding the correct heat flow through the growth cycle. Some factors at play in this balance are control thermocouple location, furnace size, crucible size, geometry of insulating refractory and heat conducting members of the furnace along with elevator position in the cycle.
For crucibles less than 3 inches diameter, the growth can be carried on without a change in the control temperature setting but for larger crystals adjustments are needed to hold the desired balance in heat flow and interface position.
The nitrogen atmosphere is maintained in the crucible throughout the entire meltdown cycle and the subsequent growth of the crystal. Typically during meltdown, a certain amount of the gas becomes dissolved in the feed stock, the amount depending upon the solubility of the gas in the melt, generally a function of molecular size of the gas molecules and the temperature of the melt. During growth, as a particular level in the melt approaches the liquid solid interface its temperature decreases, and the solubility of gas in the melt is likewise reduced. This, along with gas molecules rejected by crystal growth, creates a supersaturation of gas at the interface which can enlarge existing bubbles or even start new ones. These gas molecules must move upward in the melt or be trapped in the crystal. If the movement of the interface is faster than the upward movement of the rejected gas or if the transfer of heat through the interface is irregular and uneven, some of the gas will become entrapped in the solid crystal. This is likely to produce crystal defects which are evidenced in several ways including bubbles, bubble pipes, and blow holes, readily visible within transparent crystals, and boundary decorations. These are formed along mosaic boundaries and component boundaries. Although optically not particularly troublesome, these decorations do weaken the macrocrystal and reduce its effectiveness and utility in hot forming into shape.
Some gas inclusions in melt grown crystal are of such small size that they give an insignificant amount of light scatter. But if the ingot is hot formed by forging, extrusion or the like, they may collect into voids of objectionable size.
Prior to the present invention, none of the commercial ingots over 5 inches diameter have yielded completely clear forgings. Virtually all of these have involved component boundaries but the problem of gas inclusions also extends to mosaic boundaries that are present in all crystal grown from a melt by any of the known methods.
If it is evident that gas inclusion is occurring at the growth interface, the rate of crystal growth must be decreased to allow sufficient time for the remaining dissolved gases to diffuse away from the interface. Alternatively, other changes or adjustments must be made in the furnace equipment or in the controls to effectively slow down the growth rate. This, in turn, can create a shift in the thermal gradient and localized supercooling at the melt interface and an uneven dendritic form of crystal growth with the entrapment of dissolved gases and other impurities in the melt between dendrite arms.
Furthermore, procedural changes in crucible loading, stock preparation, heating rate and the like may eliminate or minimize the problem of bubble formation in one furnace or with one given growth stock within the furnace but will not necessarily be applicable to another furnace or another growth stock.
Prior efforts to solve this problem have been unsuccessful and no one has attached any particular significance to the role that the gases and pressures used during meltdown and growth play in the formation of bubbles within the melt and entrapment of gaseous impurity in the crystal.
In their article entitled "Growth of Alkali -- Halide Crystals in Various Gas Atmospheres" appearing in Kristallografiya 8 (6) 940-942, Nov. 1963, trans. p. 757-8, Tsal' et al describe the growing of NaI(Tl) in a helium atmosphere and find no advantage over growth in a nitrogen atmosphere. In both instances, bubbles and bubble pipes are noted.
VanSciver reported in Physics Review 120 (4) 1193, (1960) that he grew thallium activated sodium iodide in an atmosphere using hydrogen as a carrier for I.sub.2 but made no comment upon the quality of the crystal or upon any connection between atmosphere, gas pressures and growth rate.
Likewise Brinckmann in U.S. Pat. No. 3,446,745 in growing CsI(Na) uses, in Example 1, a helium atmosphere during crystal growth which proceeds at a rate of 1.4 mm per hour in 500 g. crystals, approximately 21/2 inches diameter. This is the same rate of growth that he obtains in Example 2 when using a mixture of 10% hydrogen and 90% argon.
In November, 1973, Reichelt and Stark presented a report at the Third Conference on High Power Infra-Red Laser Window Materials describing high power laser damage to alkali halide crystals, suggesting that they are possibly attributable to absorbed gases in the crystals.
Bridgmann has also commented upon troubles caused by bubble inclusions in metal ingots.
It has been noted that in smaller crystals less than three or four inches in diameter, the problem can be readily controlled. However, as larger ingots are grown, the problem of bubble inclusion becomes more pronounced and of greater economic importance. The problem has been most readily controlled by careful purification of and growth of stock to avoid insoluble solids that can start a bubble and growth at a slow rate.